The cutout capability and switch-off time of a power switch in the event of an overcurrent, a short-circuit or an accidental arc are dependent on various parameters. The switching-off process is composed of the triggering time, the inherent time and the switch-off time. The triggering time is the time from the occurrence of the variable causing the triggering to the initiation of the triggering process, for example the disengagement of a switch latch. This is followed by the inherent time of the switch, which depends on the dynamic processes of the moving and opening contacts of the switch. In the case of a switch for a relatively high power range (nominal current of up to 100 kA), switch-off times can be up to 50 to 65 msec. There are power switches which include electronic triggering means (for example, DE 19743997 A1) and allow the functional configuration of the protective conditions for a switchgear to be variably adjusted. However, the switch-off times are not altered by this electronic means.
The occurrence of an accidental arc in an electrical installation can cause serious damage to persons and/or to property; such injury and damage should be limited as much as possible if not ruled out. To avoid such damage there have been proposed various measures, of which the cut-off (the extinguishing) of the accidental arc in a time shorter than the switch-off time of the feed switch is the optimum solution. The detection of accidental arcs requires current transformers which have to be added to a switchgear installation. This takes up space and is expensive. Furthermore, additional means have to be installed to extinguish an accidental arc. The additional costs of accidental arc extinguishing means are not always financially justifiable.
A publication (“New Strategy to Counteract Arc Flash Hazards” in TANSMISSION & DISTRIBUTION CONF. and EXPOSITION: LATIN AMERICA, 2006, IEEE, pages 1-6; by Santos L. F. et al.) describes an arc monitoring system, which reacts to the occurrence of an accidental arc. The system is associated with one control means, for evaluating the accidental arc overcurrent. A turn-off pulse will be issued merely if the signal from the current detection means is above a preadjustable threshold.